Detection devices and monitoring and measurement instruments are frequently used for ascertaining the presence and concentration of selected gaseous chemical species in a subject environment. Unfortunately, in many instances, the concentration of the gas or vapor of interest may fall below the effectiveness floor of the technology in question. To address this situation, the art has devised various techniques for the concentration, or enrichment, of the gas or vapor species such that, if the species is present, it can be collected to accumulate its concentration to a level above the detection threshold of the particular technology being utilized.
One attractive conventional enrichment technique involves the use of a vapor sorbent material that serves to sorb (or “trap”) the vapor. If the sorbent material is exposed to a large volume of the vapor from the subject environment, a significant mass of the desired vapor will accumulate in or on the sorbent material. The sorbent material is then subjected to conditions which cause release of the vapor, thereby providing a collected sample of the vapor species. If the process of releasing the vapor species from the sorbent material can be accomplished in a relatively short period of time, a significant increase in the concentration of the target vapor is attained.
For example, in an apparatus for implementing this technique, vapor sorbent material is held in a tubular container. A gaseous sample is then passed through the container, wherein the vapor (if present) is trapped by the sorbent material. The container is then heated, thereby releasing a concentrated sample of the vapor, which is provided to an instrument (for example, a gas chromotograph) for analysis. Thereafter, a cooling device is used to restore the container to its starting temperature, at which time a new absorption-heating-cooling cycle can be commenced. Typically, glass wool or another wadding material is used to position the sorbent material in the container and prevent the sorbent material from moving out of its position near the heating source. However, several difficulties are associated with such apparatus.
First, the wadding material utilized to position the sorbent material does not reliably protect it against imprecise positioning (including shifting) in the vapor concentrator container, and the efficiency of the system will suffer in several respects. More specifically, imprecise positioning of the sorbent material with respect to the heating source will result in the need for extra heat and energy to attain the temperature of the sorbent material necessary for the desired rate of desorption. Consequently, the efficiency of the system will suffer as additional expenditure of energy and/or time will be required to reach the desired level of performance. Further, imprecise positioning of the wadding material can compromise the effectiveness of the vapor sorption and desorption process, leading to corresponding unpredictability in the rates of sorption and desorption of vapor species, and an inability to replace a sorbent material with one that performs comparably.
Second, the wadding material itself can interfere with the efficiency of the process. Typically, glass wool or other porous wadding material is placed in the tubular flow path of the container to maintain the positioning of the sorbent material in proximity to the heating source. While the wadding material allows the passage of vapors, it can also act as a sorbent. Thus, the wadding material must be heated to prevent high molecular weight vapors from adhering to it.
Third, a separate cooling device is generally used to assist in returning the vapor concentrator to its starting temperature, representing an additional source of power consumption. For example, the cooling device typically consists of a fan, a device that is unattractive for use in a low power system especially one having a compact design. In the alternative, passive cooling is sometimes utilized to achieve a return to the concentrator's starting temperature, but this tends to be a limited cooling solution which can extend the time necessary for the apparatus to complete a full absorption-heating-cooling cycle.
Because of the drawbacks attendant upon the above-mentioned approaches, provision of technology for securing a concentrated vapor sample, which technology is not similarly afflicted, would be a significant advance.